1. Field of the Invention
The present invention relates to a steering system for a vehicle.
2. Description of Related Art
A steering system for a vehicle is known that includes a variable gear ratio steering (VGRS) apparatus and an electric power steering (EPS) apparatus (cf., JP-A-2005-41363 corresponding to U.S. Pat. No. 7,195,098). The VGRS apparatus varies a transmission ratio (i.e., steering gear ratio) between a steering wheel angle (i.e., steering angle) of a steering wheel and a steering tire angle (i.e., steered angle) of a tire wheel (i.e., steered wheel) by using an output of a motor. The EPS apparatus assists in steering the tire wheel by using an output of another motor.
Such a steering system for a vehicle controls current conduction to a motor (i.e., a first motor) of the VGRS apparatus to manage torques output from the first motor, and varies the steering gear ratio. Further, the steering system for a vehicle controls current conduction to a motor (i.e., a second motor) of the EPS apparatus to manage torques output from the second motor, and assists in steering the tire wheel.
Such a steering system for a vehicle further includes a first electronic control unit (ECU) for mainly controlling the VGRS apparatus and a second ECU for mainly controlling the EPS apparatus. The first and second ECUs perform various control processes including processes of controlling the current conduction to the first and second motors. Recently, the first and second ECUs have been required to perform many control processes in accordance with to a high demand for vehicle traveling stability and handling stability. As a result, the first and second ECUs are subjected to much load.
In the future, such a steering system is going to perform higher-functional control processes. Accordingly, the first and second ECUs are expected to be subjected to much more load. It is therefore necessary to reduce load applied to the first and second ECUs, in order to, for example, be prepared for upcoming higher-functional control processes.